Emergency Lighting System

FIG. 1 is a schematic block diagram of an emergency lighting system of a first preferred embodiment of the invention.

FIGS. 2, 3, 4, 5 and 6 are schematic block diagrams of various alternative embodiments of emergency lighting systems utilizing the teachings of this invention.

FIG. 7 is a semi-block schematic diagram of a battery, battery charger and control circuit suitable for use with various embodiments of the invention.

FIG. 8 is a simplified schematic diagram of a feedback loop control circuit suitable for use in various embodiments of the invention including the embodiments shown in FIGS. 1 and 3.

FIG. 9 is a schematic diagram of a no-load protection circuit and of a light dimmer control being utilized in conjunction with a feedback loop control of the type shown in FIG. 8

FIG. 10 is a schematic diagram of a transistor switch control suitable for use with various embodiments of the invention including the embodiment shown in FIG. 4.

FIG. 11 is a schematic diagram of a step-down converter and inverter suitable for use with various embodiments of the invention including the embodiment shown in FIG. 4.

FIG. 12 is a schematic diagram of the feedback loop control of an inverter suitable for use with the embodiment of the invention shown in FIG. 5.

FIG. 13 is a schematic diagram of selected portions of the circuit shown in FIG. 6, including the inverter and transistor switch.

Referring first to FIG. 1, an emergency lighting system is shown which, like the systems of the related applications, is adapted to drive a first gas discharged lamp 100 from an AC source 102 through lines 104, wall control switch 106 and a standard ballast 108. As used in this application, the term "gas discharge lamp" may include, but is by no means limited to, fluorescent lamps, mercury vapor lamps and high pressure sodium lamps. AC source 102 would typically be a standard electrical line which in the U.S. would be 60 cycle and either 120 volts or 277 volts. While switch 106 will be referred to as a wall switch, it may be any switch which is utilized for controlling the turning on and turning off of lamp 100. As in the related applications, lamp 100 and additional lamp 118 may be mounted in a fixture, lamp, or in any other suitable structure.

The signal on lines 104 is also applied to the primary winding 112 of a transformer 110. Secondary winding 114 of the transformer is connected to energize a standard DC power supply 120 while secondary winding 116 of the transformer is connected to the battery charger in circuit 122. DC power supply 120 may be a standard power supply for converting the line signal input, the voltage of which has been stepped down by transformer 110, to a signal of suitable DC voltage on lines 124 to operate DC/AC inverter 126. Inverter 126 may be a standard self-oscillating inverter having a feedback loop, the oscillating frequency of the inverter being determined in part by its load. The output from inverter 126 is connected through a no-load protection circuit 128 to operate gas discharge lamp 118. A starter strip 130 is provided to reduce, in a well-known fashion, the potential required to fire lamp 118, thus permitting an inverter 126 generating a single voltage output level to be utilized. If the starter strip was not utilized it might be necessary to use a more complex and expensive inverter to produce two separate voltages: a higher voltage to start the lamp and a lower voltage to operate it.

When battery charging voltage is being applied to circuit 122 from windings 116, a signal is applied over line 132 to battery transfer switch 134 This signal indictes that AC voltage is available from source 102. An output signal from the battery in circuit 122 is applied through line 136 to battery transfer switch 134. As will be described in greater detail hereinafter, battery transfer switch 134 is operative to pass the battery signal on line 136 to line 138 when the switch detects that signal is not present on line 132 (or is reduced below a specified threshold), indicating that there has been a loss (or the reduction below a specific threshold) of AC voltage from source 102. The signal on line 138 is applied through lines 124 to inverter 126 to energize lamp 118 and, if necessary, starter strip 130, in lieu of the signals from power supply 120. The signal on line 138 is also applied to low battery voltage shut-off circuit 140, which circuit, as will be described in greater detail later, is operative when the battery output voltage drops below a predetermined threshold, to generate an output signal on line 142 which is operative to open or disable switch 134, preventing further drain on the battery, thus preventing the battery from becoming overdischarged. Battery voltage on line 136 is applied to energize low-voltage circuit 140 and to also energize various control devices for the system.

More particularly, a number of controls are provided for the lamp 118 to permit it to be lit under selected conditions and to disable it under other selected conditions so as to avoid excessive drains on the battery. As previously indicated, inverter 126 has a feedback loop as part of its self-oscillating circuitry. Feedback loop control 146 represents a control circuit in the feedback loop of the inverter which is operative to close the feedback loop to permit the oscillator to generate a desired output or to partially or fully interfere with the operation of the feedback loop to reduce or eliminate output from the inverter The manner in which this circuit operates will be described in greater detail hereinafter. For purposes of the present discussion, it will be noted that there are two types of inputs to feedback circuit 146. The first type of input enables the inverter to convert the DC input on line 124 to an AC output to drive lamp 118. These controls include wall switch sensor 148 which determines that there is an AC signal on lines 104 at a point beyond switch 106, and thus that switch 106 has been closed. The AC input to switch 148 is isolated by the switch sensor to produce a DC output on lines 150 to feedback control circuit 146, enabling the inverter, when AC power is present and switch 106 is closed.

Similarly, battery transfer switch 134 generates a DC output on line 152 when the transfer switch is enabled to pass battery voltage to the inverter The signal on line 152 enables the inverter to operate with battery voltage input.

It is noted that in the circuit as shown in FIG. 1, switch 106 has no effect on lamp 118 when the circuit is operating in the battery mode. In many applications, this is a desirable feature since it assures that there will be emergency lighting without the need to grope for a switch when there is a power outage. However, this also results in a drain on the battery in circuit 122 during intervals when the user may have no need for emergency lighting and may in fact not even be on the premises. Thus, a dotted line 152' is provided which is an alternative connection for the line 152. Line 152', instead of being connected as an input to feedback circuit 146, is connected on the side of switch 106 opposite switch sensor 148. Thus, the output on line 150 may be caused either by AC signal from source 102 or DC signal from line 152 if switch 106 is closed. The problem with the alternative configuration for line 152 is that, in many areas, building codes prohibit running both an AC signal and a DC signal through the same switch In such areas, other means, some of which will be discussed later, may be provided for permitting a switch to control lamp 118 in both the AC and DC modes.

An additional input to feedback control 146 which causes lamp 118 to be illuminated may be output lines 154 from people and intrusion detector 156. While the specific nature of the circuit 156 does not form part of the present invention, and any one of a number of commercially available products may be utilized to perform this function, it has been found that such devices operate far more reliably if they employ two or more methods of detecting an individual rather than a single method. Thus, the inputs to detector 156 might include an infrared detector 158 and one or more ultrasound detectors, such as microphone(s) 162, broadcast by a speaker 160. Where circuit 156 is being used as an intrusion detector, the output signal from this device would be utilized to turn on the lamp when the lamp is not otherwise on, either in the AC or DC mode. Thus, even though switch 106 was open, so that no signal was present on line 150, the signals on line 154 would be operative to turn on lamp 118, alerting occupants that an intruder was present and providing illumination so that the intruder would be visible. When operating in the DC mode, detector 156 may also be utilized to conserve battery power when lamp 118 is, for example, being used to illuminate a door, hall or other source of ingress or egress. In such circumstance, the detector would serve as a sink in a manner to be described later to prevent illumination of lamp 118 until a person is detected in the area being illuminated. At such time, the sink signal from detector 156 would disappear, permitting lamp 118 to be lit from inverter 126 under control of the signal on line 152.

Remote input terminals 164 are also provided which may provide signals over line 166 in response to any desired input. For example, terminals 164 could be connected to the output of a smoke detector, a heat detector, a water or dampness detector, or some other sensor device which would indicate an emergency condition where extra lighting might be required. The turning on of lights under such an emergency might also alert the occupant of potential trouble.

The second type of input to feedback control circuit 146 are inputs which override previously received turn-on signals and disable the inverter to turn the lamp off when certain selected conditions occur, thereby conserving battery power. Examples of the second type of control include ambient light detector 168 which is connected to the feedback control circuit through lines 170. When ambient light exceeding a predetermined threshold is detected by sensor 172, circuit 168 becomes a DC current sink for control signals applied to feedback control circuit 146 and is thus operative to disable the inverter, causing lamp 118 to be turned off. Assuming the other control inputs are still present, the inverter 126 would become enabled again when sensor 172 indicated that the ambient light had fallen below the indicated threshold.

Similarly, a manual dimmer control 174, such as a rheostat, may be provided which is operative through circuit 176 and lines 180 to serve as a DC current sink for all or a portion of the turn-on signals to feedback control circuit 146. With dimmer control 174 partially down, the voltage output from inverter 126 will be reduced, reducing the drain on the battery and reducing the illumination output level from lamp 118. Control 174 may be turned all the way down, causing sufficient current to be drained from the feedback control loop to disable inverter 126, thus turning off lamp 118. Control 174 may thus serve as a DC control for lamp 118 when the lamp is operating in its DC mode and in this manner may perform substantially the same function as was performed by the line 152'. This, however, requires that there be two control switches for the lamp rather than one.

While there are various differences in the circuitry for the embodiments of the invention to be described hereinafter, all embodiments use the same battery and battery charger circuit 122, battery transfer switch 134 and low battery voltage shut-off circuit 140. These circuits have therefore been enclosed in a dotted box 184 which will be designated the battery, battery charger and control circuit in the remaining embodiment. Similarly, the various auxiliary controls within dotted box 186 may also be used with all of the embodiments of the invention to be described hereinafter. A single block labelled "Auxiliary Controls" is therefore shown in the drawings for the remaining embodiments which block may contain any one or more of the controls within the block 186 in FIG. 1.

FIG. 2 shows an embodiment of the invention which is similar to that shown in FIG. 4 of the beforementioned Ser. No. 933,266 application, but incorporating some of the added features of this invention. The embodiment of FIG. 2 differs from that shown in FIG. 1 in that the control circuit for inverter 126 is from a solid state switch isolator and control circuit 200, rather than from the feedback loop control circuit 146. Circuit 200 is connected in series between the source of DC power on lines 124 and inverter 126, and is operative to either pass DC power to the inverter or block the passage of DC power to the inverter, depending on the control inputs to the circuit on lines 147, 152 and output lines 202 from auxiliary controls 186. The various controls function to either enable or disable inverter 126 in substantially the same way previously described for the embodiment of FIG. 1. While the embodiment of FIG. 2 is an operative one and one which may be utilized in some applications, it is not considered as desirable as the embodiments shown, for example, in FIGS. 1 and 4, because the circuit 200 at the input to the inverter causes a greater energy loss in the control circuit than does a control circuit placed in the feedback loop or the output of the inverter, thus resulting in a lower efficiency circuit and more heat. As previously indicated, an objective of this invention is to minimize the heat generated by the various components so as to hold down the ambient temperature for the battery.

FIG. 3 shows an alternative embodiment of the invention which is similar to that shown in FIG. 5 of the beforementioned application Ser. No. 933,266 in that both lamp 100 and lamp 118 are being driven from inverters 126 in the AC mode rather than from a ballast. For purposes of illustration, one of the inverters, inverter 126A, is shown as being controlled by an isolation and control circuit 200 of the type shown in FIG. 2, while the other inverter, inverter 126B, is shown as being controlled from a feedback loop control circuit 146. The auxiliary controls 186 on lines 202 are shown connected to both control circuits. However, depending on the application, different controls may be applied to each of the control circuits and, in particular, since only lamp 118 operates in the DC mode, controls such as ambient light detection, which are designed to converve battery, may only be utilized with circuit 146 and inverter 126B.

The second major difference between the circuit of FIG. 3 and the circuits of the prior embodiments is that transformer 110 and DC power supply 120 have been replaced with a step-down converter DC power source 300, which generates the signals on line 124 to the inverters and also generates a DC charging signal for the battery on line 302. Circuit 300 which may be a standard circuit for performing this function, may be preferable to the transformer 110 and DC power supply 120 in that it operates at a higher frequency than the prior circuits, for example, 20-25 kh, and is therefore more efficient than such circuits, being smaller for comparable ratings and generating less heat.

Except for the distinctions indicated above, the circuit of FIG. 3 operates in all respects in the same manner previously described with respect to the circuit of FIG. 1.

FIG. 4 shows another embodiment of the invention which differs from previous embodiments in two respects. First, the step-down converter DC power source 300 has been replaced with a combined step-down converter and inverter 400 which receives AC power from line 104 through a surge and transient protection circuit 402. The circuits 400 and 402, which will be described in greater detail later in connection with the description of FIG. 13, convert the AC line signal on line 104 into a low voltage signal on line 404 which is applied to circuit 122 to charge the battery and a high frequency AC signal which is applied through no-load protection circuit 128 and transistor switch 406 to operate lamp 118 and starter strip 130. The use of a step-down converter for this embodiment of the invention has the same advantages over the use of a transformer and DC power supply previously discussed in the connection with the embodiment of FIG. 3. However, by combining the converter and inverter in the same circuit, the circuit provides the desired AC signal without multiple conversions. This results in a significantly more efficient circuit which is less expensive, more compact, provides more output for the same energy input and generates less heat. Surge and transient protection circuit 402 is required in this embodiment of the invention to prevent spikes, ripples and the like in the AC line signal from being applied to and burning out circuit 400, and in particular the power transistor(s) therein.

The second difference between the embodiment of FIG. 4 and prior embodiments is that the isolation and control circuit in the form of transistor switch 406 is in the output of the inverter rather than in its input. The switch 406 along with wall switch sensor 148 for this embodiment of the invention are shown in FIG. 10 and described in greater detail in conjunction therewith. This circuit basically functions in the same manner as the circuit 200 in FIG. 2 to pass the AC signal to the output under control of the various control inputs. However, since this switch operates at a much higher frequency and at a much higher voltage than the switch circuit 200, it operates at much lower current and the power loss in this circuit is thus substantially less than for the circuit 200. The circuit is thus more efficient than the circuit 200, providing greater output and less heat for the same input.

FIG. 5 shows still another embodiment of the invention which differs from prior circuits shown in that a ballast 500 is provided to operate lamp 118 when it is operated in the AC mode. Thus, the lamps 100 and 118 operate in substantially the same manner in the AC mode. Since lamp 118 is operated from ballast 500 in the AC mode, a power supply from transformer 110 is not required to drive inverter 126 in this mode, the inverter only being utilized to drive lamp 118 in the DC mode. Thus, transformer 100' has only a single secondary winding which is connected to circuit 122' to charge the battery.

Further, since the inverter is not being used in the AC mode, an inverter holdoff circuit 502 is provided which is operative when an AC signal is present on lines 104 to serve as a DC current sink preventing any spurious turn-on of the inverter.

One potential problem with the circuit shown in FIG. 5 is that spikes, ripples or other undesired signals on the AC line which are passed by ballast 500 may be applied back to overload and thus burn out or otherwise damage inverter 126, and in particular the power transistor therein. In the prior art, this problem has been dealt with to some extent by either having relay contacts which open the circuit to the inverter when power is being applied through the ballast or by attempting to filter such undesired signals For reasons which have been discussed previously, neither of these solutions has proved satisfactory. Thus, as will be discussed in greater detail in conjunction with FIG. 12, the feedback circuit 146 and the transformer secondary circuit of the inverter are designed to provide a lossy load for undesired ballast signals and to assure that any such signals which do reach the power transistor of the inverter are of a polarity to turn the transistor off and well below its breakdown potential.

Except for the differences discussed above, the circuit of FIG. 5 operates in the manner indicated above to control the turning on and turning off of lamp 118 in response to various external controls.

FIG. 6 shows still another embodiment of the invention which differs from the embodiment shown in FIG. 5 only in that the isolation and control circuit consists of a switch sensor and a transistor switch 400' rather than the feedback control circuit. Circuit 400' is of the.general type utilized in conjunction with the embodiment of FIG. 4 and performs substantially the same functions as performed by these components in FIG. 4. The main difference between the embodiment of FIG. 6 and that of FIG. 4 is that the inverter is utilized only in the DC mode, not the AC mode, and thus an inverter holdoff circuit 502 is provided Circuitry for the sensor 148' and transistor switch 400' are shown in FIG. 13 and the manner in which these circuits operate, including how they operate to protect the inverter against spurious AC signals will be discussed in greater detail in conjunction with FIG. 13.

FIG. 7 is a schematic diagram of the battery, battery charger and control circuit 184 including battery transfer switch 134 and low battery voltage shut-off circuit 140. When AC signal is present, there is a charging signal on line 700 from, for example, winding 116 (FIG. 1) or step-down converter 300 (FIG. 3). The charging voltage passes through a parallel network consisting of zener diode 702 in one leg, and light-emitting diode 704, resistor 706 and diode 708 in the other leg. LED 704 provides a visual indication that the circuit is on. The fixed drop across zenor diode 702 assures that the potential at the base of transistors 710 and 712 are sufficiently above the battery potential at the emitters of these transistors so as to bias the transistors off. For example, the battery voltage would typically be 12 volts while the charging voltage might be 13.5 volts. Thus, so long as AC charging voltage is present, there is no DC signal applied by battery 714 to line 138, and there is no enabling control signal applied to line 152 from transistor 712. The DC mode is thus totally disabled.

Circuit 715 functions to charge the battery and to protect the battery against being overcharged. One circuit for performing this operation has been described in the related application. Other standard circuits for performing this function are also available.

When there is a loss of AC power so that a charging signal no longer appears on line 700, the potential ar the base of transistors 710 and 712 drops below the battery potential, causing both of these transistors to become conductive. This results, as previously indicated, in battery potential being applied through line 138 to the inverter (or to the switch for embodiments such as FIG. 2) and in a control signal being applied through transistor 712 to line 152. Signals will remain on lines 138 and 152 until either an AC charging signal again appears on line 700, cutting off transistors 710 and 712, or the battery drops below a predetermined threshold value, for example 85 percent of its rated potential, in which case low battery shut-off circuit 140 generates an output which turns off transistors 710 and 712. More particularly, when the circuit is operating in the battery mode, transistor 716 is conducting and transistor 718 is cut off, maintaining the basis of transistors 710 and 712 at a low enough potential for these transistors to remain conducting. When the battery potential drops below a selected value, the potential drop across resistor 720 is not sufficient to keep transistor 716 conducting. This transistor thus becomes cut off, turning on transistor 718. This raises the potential at the basis of transistors 710 and 712 to prevent further drain on the battery.

FIG. 8 is a simplified schematic diagram of the inverter 126, switch sensor 148 and related circuitry for an embodiment of the invention such as that shown in FIG. 1. For this embodiment of the invention, when wall switch 106 is closed, an AC signal from source 102 appears on line 147 which causes the light emitting diodes 800 in opto-isolator 148 to illuminate. The illumination of LEDs 800 causes current to flow in phototransistor 802, the emitter of which is connected to the base of transistor 804 to turn this transistor on. The output from transistor 804 is connected to energize the power transistor 806 of the inverter The feedback loop of the inverter consists of transformer primary winding 808, transistor 806, capacitor 810 and diode 812 connected in parallel, feedback winding coil 814 and resistors 816 and 818.

In addition to being turned on by the output from phototransistor 802, transistor 804 may also be turned on by a signal on line 152 from battery transfer switch 134 or by a signal on one of the lines 820 forming a part of cable 202 from an auxiliary control such as remote input terminal 164 or intrusion detector 156 which causes the light to be turned on. Thus, even though switch 106 is open, the inverter may still be enabled to convert power from DC power source 120 to operate lamp 118 if a selected emergency is detected.

Similarly, lines 822 of cable 202 lead to control devices which provide a DC current sink for control signals on line 202, causing transistor 804 to be cut off regardless of the presence of turn-on control signals on line 147, line 152, or line 820. Examples of devices serving as DC current sinks include light dimmer 176, ambient light detector 168 and possibly a people detector 156 in certain modes of operation previously described.

FIG. 9 is a schematic diagram showing among other things a light dimmer circuit suitable for use as the circuit 176 and how this circuit interacts with the inverter feedback controls to serve as a DC current sink. Capacitor 900 provides a measure of the voltage across lamp 118 which measure increases with the increase in frequency in the self-oscillating inverter output when lamp 118 is lit. This measured voltage is polarized by diode 902 and applied through a voltage divider network consisting of resistors 904 and 906 and rheostat 174 to the base of transistor 908. With the rheostat in its fully on position, i.e. in the position for the lamp 118 to be fully lit, transistor 908 is fully cut off preventing the flow of current therethrough. As the rheostat is turned down, the resistance of rheostat 174 increases, causing the drop across the rheostat to increase and thus increasing the potential at the base of transistor 908. This causes the transistor to become increasingly more conductive, draining current from cable 200 through diode 910, transistor 908 and resistor 912. This reduces the current flowing through the feedback loop and thus the output from the inverter, thereby dimming lamp 118 and reducing the load on the battery. Resister 912 permits transistor 908 to have a high input impedance when lamp 118 is not lit so that the dimmer does not load the circuit during the startup process.

FIG. 9 also illustrates the circuit for operating starter strip 130 and the no-load protection circuit. The potential on starter strip 130 is determined by resistor 916 and capacitor 918. Starter strip 130 is positioned adjacent to lamp 118 and is capacitively coupled thereto. During startup, the potential on strip 130 is midway between the potential across lamp 118, reducing, in a well known manner, the potential required to ignite the lamp.

The no-load protection circuit consists of capacitors 900 and 920 which are connected in series with resistor 922 therebetween, this series network being connected in parallel with lamp 118. The values of the capacitors are such that, at the high operating frequency of the fluorescent lamp, the circuit provides a sufficiently high impedence to the AC signal that little energy is lost through this circuit. However, if lamp 118 fails or is removed, the self-oscillating inverter oscillates at the higher frequency of the no-load circuit, permitting this circuit to provide a load to the inverter. As previously indicated, if a self-oscillating inverter does not have a load, there is a possibility that it will destroy itself. The no-load protection circuit also limits the voltage at the socket for lamp 118, protecting the user against shock from touching the socket during tube insertion or removal.

FIG. 10 shows a transistor switch suitable for use as the transistor switch in various embodiments of the invention where the transistor switch is in the output of the inverter, such as, for example, FIG. 4. For this embodiment of the invention, the AC signal from switch 106 on line 104 is connected through lines 147 to illuminate two pairs of light emitting diodes 930A and 930B. Thus, during each half cycle of the AC signal, when AC signal is present, one LED of each pair is illuminated to energize the corresponding phototransistor 932 to turn on the corresponding switch transistor 934. Thus, when AC signal is present, the switch is open to pass AC signal from the inverter to energize lamp 118.

Control signals may also be applied to the switch through line 202 and diode 936 to energize the switch. As previously discussed, some of the controls attached to line 202 may in fact be DC current sinks which, when present, cause lamp 118 to be turned off by serving as a DC current sink to the signals which would otherwise cause LEDs 930 to be illuminated. However, as can be seen in FIG. 10, in view of diode 936, these signals are not operative to prevent the illumination of lamp 118 when AC signal is present.

FIG. 11 shows a portion of the feedback control circuit and inverter output circuit for the embodiment of the invention shown in FIG. 5, and is included to illustrate how this embodiment of the invention functions to protect the power transistor 806 in the inverter from AC signals, and in particular, spike and ripple AC signals which may be applied from the AC line and ballast 500 through the inverter transformer to the transistor. The primary means for suppressing such signals is to provide a lossy path to ground for such signals through capacitor 916 and resistor 918. Since these components in parallel with the main AC circuit used to drive lamp 118, they do not load this circuit. Further, capacitor 916 is selected to provide a relatively higher impedance at the 6 to 10 KHz spike and ripple signals from ballast 500 thus causing most of these signals to pass through resistor 918. At the higher frequencies at which the lamp operates, i.e. above 20KHz, capacitor 916 is substantially a short circuit reducing the loss caused by this circuit when the system is in DC mode. Further, these components are designed to present a higher load path to ground for the higher frequency signals out of the inverter when the system is in the DC mode, so that they do not adversely affect the performance of the circuit in this mode.

Capacitor 950 tends to block any lower frequency signals, for example signals at 60 cycles and below from the AC line and prevent such signals from being applied to the inverter transformer. Higher frequency signals are routed through capacitor 950 and secondary coil 952 of the inverter transformer to ground through capacitor 916 and resistor 918.

To the extent signal is transmitted through the secondary coil of the transformer to the primary coil 808, it is applied through resistor 818 and resistors 816 to transistors 804 and diode 956 connected in parallel therewith. Diode 956 blocks the passage of signal in the direction to turn on transistor 806 from being transmitted to the transistor and passes the ballast signal in the direction to turn transistor 806 off. The breakdown potential of transistor 804 is greater than that of transistor 806, so that any back bias potential applied to transistor 806 will not be sufficient to cause any damage thereto. When opto-isolator transistor 802 is turned on it shorts to ground the current in the polarity to turn transistors 804 and 806.

The ballast signal through the secondary coil 814 from 952 is shorted by the opto-isolator transistor 802 is shorted to ground when AC power is present. The circuit is thus effective to completely protect power transistor 806 against any possible damage from the ballast without the use of relays to open the circuit.

FIG. 12 is a partial block schematic diagram of a portion of the circuit shown in FIG. 6 which illustrates the operation of the transistor switch 400' for this embodiment of the invention, and also illustrates how this embodiment of the invention functions to protect power transistor 806 of the inverter against damage from spurious signals applied from the AC line from ballast 500, including any spikes or ripples on such line.

Unlike the transistor switch shown in FIG. 10, the transistor switch in FIG. 12 has a transistor control switch in only one direction, and has a diode 970 in the other direction. This means that when a sink signal is received from, for example, the ambient light detector or the light dimmer, these signals are effective to prevent the passage of signal to the lamp during only one half a cycle. Normally, the absence of signal during a full one half cycle will be sufficient to cause the lamp to be cut off, although it might take several cycles for this to occur. This permits a reduced number of components in the switch, and thus makes the switch simpler and less expensive.

The change in the transistor switch also means that there is only a diode 970 in the circuit in one direction. As with the embodiment of the invention discussed in conjunction with FIG. 11, capacitor 950 is effective to block all but higher frequency signals from the AC line and ballast 500, for example, signals in excess of 60 cycles. Further, diode 970 is effective to block any transient signals which might occur in the circuit which would tend, when projected through the inverter transformer, to turn power transistor 806 on. Thus, the signals projected back through the transformer, are only in a direction to turn power transistor 806 off. Capacitor 916 and resistor 918 serve in the same manner discussed with respect to FIG. 12 to provide a moderately lossy path to ground for the spurious signals from ballast 500. As previously indicated, with such signals, which are generally in the 6 to 10 kh range, most of such signal passes through resistor 918. Conversely, at the frequency at which the lamp operates capacitor 916 provides a substantial short circuit, providing very little loss across resistor 918, so that the circuit is not loaded by these components when the inverter is operating the lamp in the DC mode.

FIG. 13 illustrates a circuit such as that shown in FIG. 4, including in particular details of the surge protection circuit 402 and step-down converter and inverter circuit 400 used therein. Referring to FIG. 13, it is seen that signals from AC source 102 are applied through diodes 1000 and 1002 respectively, which diodes are poled in opposite directions, to primary coils 1004 and 1006 of transformer 1008 in the converter/inverter 400. There are two feedback loops provided for the self-oscillating inverter portion of the circuit. Each path includes a feedback coil 1010, a resistor 1012 connected in parallel with a capacitor 1014, a power transistor 1016, which is connected at its collector to one side of a primary winding and a diode 118 and capacitor 120 connected in parallel. A varister 1022 is connected across AC source 102 and serves as the surge protector 402.

The converter/inverter 400 serves two functions, one being to rectify the AC input to provide a signal on line 404 to charge the battery, and the other being to step up the frequency of the AC signal to the frequency necessary to drive lamp 118. The first function is accomplished through output coil 1024 which is rectified by diode 1026. The output from diode 1026 is passed through transistor 1028, which transistor is turned on to pass signal to line 404 when there is AC signal on line 104. The AC signal on line 104 is passed through diode 1030 and resistor 1032 to the base of transistor 1028.

The second function is performed by secondary coil 1034 which is connected through no-load protection circuit and a transistor switch which are substantially the same as those shown and described with respect to previous figures to energize lamp 118.

When AC signal is not present, battery transfer switch 134 passes a DC signal output on line 138 which is applied to primary coil 1036. Since AC signal on line 104 is not present at this time, transistor 128 is cut off, preventing battery charging current from being applied to the battery, and thus avoiding an extra drain on the battery The DC signal applied through coil 136 provides energy to the tank circuits which cause an signal on coil 1034 at the desired AC frequency.

An emergency lighting system has thus been described which is adapted to provide light as desired when AC signal is present, to provide backup lighting from a DC battery source when AC signal is not present, and which has the capability to turn on at least one lamp under selected emergency conditions, and to cause the light to be turned off to conserve battery power under other selected conditions. While for the various embodiments of the invention described above, two lamps have been shown, one of which operates only in the AC mode and the other of which operates in both the AC and DC modes, it is apparent that the teachings of this invention could be practiced with only a single lamp which operates in both the AC and DC mode, and that additional lamps could be provided which operate only in the AC mode or in both modes. Limitations on the number of lamps which can be employed include space in the fixture, heat which can be dissipated from the lighting device so that the lights do not cause excessive ambient temperature which might damage the battery, and the energy capacity of the battery to illuminate lamps in the DC mode.

While the invention has been shown and described above with respect to a number of different embodiments, it will be apparent to those skilled in the art that the foregoing and other changes in form and detail may be made therein by one skilled in the art without departing from the spirit and scope of the invention.